1. Field of the Invention
This invention relates to the separation of gaseous mixtures by pressure swing adsorption systems that utilize a plurality of primary adsorbent beds and at least one auxiliary secondary adsorbent bed. More particularly, this invention relates to the separation of gases wherein there are at least two primary adsorbent beds and at least one auxiliary secondary adsorbent bed, the auxiliary secondary adsorbent bed being interconnected to the adsorbent bed undergoing regeneration.
2. Description of Related Art
One of the various processes for the separation of gaseous mixtures is through the use of pressure swing adsorption. In a pressure swing adsorption system, one or more gases will be preferentially adsorbed by an adsorbent contained in a chamber. Other gases in the mixture are not so preferentially adsorbed and pass through the adsorbent-filled chamber. The desired product gas or gases can be those that are preferentially adsorbed or those that pass through the adsorbent chamber. Upon the adsorbent in a chamber being saturated with the preferentially adsorbed gas or gases, the flow of gas into the chamber ceases and the adsorbent in the chamber undergoes one or more regeneration steps. At this point, the preferentially adsorbed gases are adsorbed on the adsorbent and the void space between the adsorbent particles is filled with gas of essentially input feed gas composition. During regeneration the preferentially adsorbed gas or gases are removed from the loaded adsorbent. The techniques and processing steps used to remove the preferentially adsorbed gas or gases and the degree to which they recover the gases in the void space will determine the efficiency of the pressure swing adsorption system.
Pressure swing adsorption systems usually are comprised of two or more adsorbent beds with an adsorbent bed being a quantity of adsorbent contained within a chamber. In most instances two to four adsorbent beds will be used. But there are systems that use more than four adsorbent beds, and there are single adsorbent bed systems. However, two to four adsorbent beds appear to provide cost advantages when both capital costs and operating costs are considered.
The adsorbents that are used in these pressure swing systems can be wide-pore adsorbents or they can be molecular sieves. A wide-pore adsorbent has wide pores and will admit most or all of the components of a gaseous mixture and preferentially attract and retain one or more of the components. A molecular sieve is a material that has pores of a specific size through which one or more of the components of the gaseous mixture will pass and then be adsorbed within the pore structure. The components of the gaseous mixture that will pass into the pores of the adsorbent are of a molecular size equal to or less than the pores of the adsorbent. In these adsorbents there is a sieving and an adsorbing.
The present invention is directed to pressure swing adsorption systems having a greater efficiency. These pressure swing adsorption systems can use wide-pore adsorbents or molecular sieve adsorbents. Although the pressure swing adsorption systems can be used for many different gaseous separations, the pressure swing adsorption systems are very useful for the preferential adsorption of carbon dioxide and the removal of carbon dioxide from hydrocarbon containing streams. In such separations a molecular sieve adsorbent preferably is used.
The systems of the present invention use a plurality of primary adsorbent beds and at least one secondary adsorbent bed. The secondary adsorbent bed does not receive the feed gas stream to the pressure swing adsorption system, but rather is interconnected to a primary adsorbent bed while that adsorbent bed is undergoing regeneration. By the use of a secondary adsorbent bed during regeneration, there is a greater recovery of the gases that are not preferentially adsorbed. When two primary adsorbent beds are used, there will be an intermittent production of the less preferentially adsorbed gases while in the use of three or more primary adsorbent beds there can be a continuous production of the less preferentially adsorbed gases from the system.
The feed gas to the pressure swing adsorption system can be a gas with little or no prior processing or can be a gas that has been processed, as for instance in a membrane separation unit. If the feed gas to the pressure swing adsorption system is a gas from a membrane separation unit it usually will be the permeate gas stream. However, it could be the non-permeate residual stream. In such an instance where the feed gas to the pressure swing adsorption system is the permeate stream, the non-permeate stream usually will be rich in a primary product gas while the permeate feed gas usually will be rich in a secondary product gas and contain some primary product gas. The primary product gas and secondary product gas will then be separated in the pressure swing adsorption system. The separated primary product gas from the pressure swing adsorption system usually will be compressed and combined with the non-permeate residual gas from the membrane separation unit. When the feed gas to the pressure swing adsorption system is the permeate stream from the membrane separation unit, the pressure swing adsorption system can be sized smaller since a portion of the input feed has been removed as residual gas.